Microscopes used in biological applications typically place a cover glass over a specimen to isolate the specimen from the atmosphere. Such cover glass typically has a thickness of approximately 0.17 mm. The high numerical aperture dry objective lenses typically used in biological microscopes may be designed to compensate for specified thicknesses of cover glass. However, if the thickness of the cover glass varies from the thickness specified in the lens design, fixed lenses are still susceptible to spherical and other. aberrations which impair resolution and contrast.
In order to compensate for variations in cover glass thickness, several microscope vendors now produce cover glass correcting objective lenses for biological microscopes. These lenses allow adjustment for variations in cover glass thickness to ensure the optimum objective performance. For example, Nikon Instruments Inc. manufactures the “CFI Plan Apochromat 40×C,” which has a numerical aperture of 0.95 and corrects for cover glass thicknesses from 0.11 millimeters to 0.23 millimeters.
Mireau interference microscopes, which have applications in surface metrology, inspection of microfabricated parts, and metrology of semiconductor wafers and masks, typically are not used with a cover glass. However, they do include thicknesses of glass used to support the beam splitter and mirror. If the combined glass thickness is more than about 1000 Angstroms, spherical aberration are introduced into the microscope image. For small numerical apertures this effect is small. However, the effect increases for larger numerical apertures, and is significant for numerical apertures greater than 0.7.
An example of the background and prior art includes:
U.S. Pat. No. 5,073,018December, 1991Kino et al.359/368
The Kino et al patent teaches a Mireau interference microscope having a very thin beamsplitter (less than 1000 Angstroms) in the high numerical part of the beam of an objective lens. Kino et al also teaches a method of making such the thin beamsplitter.
A general reference on Interference microscopy is: Harihan, P., Optical Interferometry Second Edition, Academic Press, Amsterdam, Second edition, 2003.
However, it is difficult to fabricate support glass for the beamsplitter and mirror whose thickness is less than 1000 Angstroms and such thin parts are also excessively fragile.
It is desirable to provide a Mireau interference microscope having support glass structures for the beam splitter and mirror which have a combined thickness of approximately 200 microns and to correct for the resulting aberrations.